The invention relates to a plant for continuously preparing starch glue by gelatinizing a primary starch fraction and mixing the resulting carrier with a secondary fraction from ungelatinized starch, which plant comprises a reactor which is divided by partitions into adjacent cells having each a cylindrical inner wall, which cells communicate together with a limited passage opening, a stirrer arranged inside the reactor with a shaft extending through the passage openings and a stirring disc mounted inside each cell on, said shaft and a drive mechanism for said stirrer, which reactor comprises an inlet for the primary starch fraction and an inlet for a product for gelatinizing said primary starch fraction, which inlets open in a cell lying at the one reactor end, an outlet for the glue which outlet connects to the cell lying at the other reactor end, and an inlet for the secondary fraction which inlet opens in an intermediate-lying cell.
Known plants for continuously preparing starch glue or paste, among others in the corrugated cardboard industry, comprise two discrete mixing volumes, namely a first mixing volume wherein a starch suspension is mixed with caustic soda, whereby gelatinization occurs and the carrier is formed, and a second mixing volume wherein said carrier is mixed with a suspension from native starch and borax, whereby the glue or paste is formed. Both mixing volumes are each fitted with a separate stirrer driven at a different speed.
When preparing the carrier, use is always made of a relatively low stirring speed, generally but 1500 r.p.m. With a higher speed there always appears a temperature increase resulting from mechanical shearing forces, whereby the temperature inside the mixing volume may rise up to 60.degree.. Such a high temperature results when contacting the primary starch fraction with the secondary starch fraction inside the second mixing volume, in an additional gelatinization of said secondary starch fraction. In the primary starch fraction or carrier an excess sodium hydroxide is always present which can cause gelatinization in the native starch, at the above mentioned high-temperature. Such additional gelatinization of the secondary fraction results in a premature gelatinization of the glue and is thus undesirable.
When mixing the carrier and the secondary starch fraction inside the second mixing volume to the contrary, the abovementioned relatively low stirring speed, for example 1500 r.p.m., is not high enough to obtain a homogeneous mixing. Use is then generally made also of a speed of 3000 r.p.m.
It is indeed due generally to the requirement for a different mixing speed that the known plants comprise at least two discrete mixing vats.
Both mixing volumes are generally open vats with a relatively large content, namely between 100 and 500 liters. The mixing occurs thus at atmospheric pressure. The content, the shape and the equipment of the first mixing vat are so chosen that the sodium hydroxide and the starch remain long enough in contact together with a high enough concentration and at a determined temperature for the gelatinizing of the starch and the homogenizing, while the volume of the second mixing vat is so selected that an optimum homogenizing and stabilizing is possible.
Said known plants do not only have the drawback of being relatively voluminous due to the requirement for two relatively large mixing vats, but such plants also are not very flexible. Such plants require relatively much time to provide usable glue after starting the mixing. Moreover, inside each mixing vat is present a relatively large amount of product, in such a way that should the plant have a failure, relatively a lot of product will be lost.
To obviate already somewhat the above-mentioned drawbacks and mostly to increase the production capacity, there have been designed plants which comprise more than two mixing vats and generally have four mixing vats. Such mixing vats are arranged in series, they have a relatively small content, generally but 2.5 liters, and they are completely closed. Inside a first mixing chamber, the starch suspension is by means of caustic soda. The homogenizing and stabilizing of the thus resulting carrier occurs inside a second mixing chamber. Inside a third mixing chamber, the homogenized and stabilized carrier is contacted with the secondary starch fraction. To obtain a stable glue or paste, the resulting mixture should further be stirred, which occurs inside a fourth mixing chamber and according to the throughput of the plant, possibly also inside following chambers.
Such a plant with four or more closed mixing chambers is naturally relatively intricate and expensive, notably due to each such mixing chambers having its own stirrer and own driving mechanism therefor. The stirring speed when preparing the carrier is always different from the stirring speed when mixing the carrier and the secondary starch fractions and thus preparing the glue, which makes any automatizing rather delicate.
In Belgian Patent No. 809.123 in the Applicant's name, which pertains to such plants, it is actually mentioned that two or four mixing vats may be replaced by a multi-cell reactor, but with the multicellular reactors known up to now, no glue with satisfactory properties could be obtained. In such a multi-cell reactor, the stirring in all of the cells occurs with the same speed, while actually the carrier preparation and the glue preparation should occur with a different stirring speed.
The invention has now for object to obviate all said drawbacks and to provide a plant for continuously preparing starch glue of good quality, in which plant the mixing vats are replaced by a compact multicellular reactor of simple structure.